Printer or copier for simultaneously printing a supporting material on both sides

ABSTRACT

A printer or copier operates to print a carrier material on both sides. A transfer station in the printer or copier has two transfer bands which transfer toner images onto the supporting material. A transfer corotron recharges the toner particles to a polarity for transfer to the carrier material at a transfer location. A first transfer mode provides for accumulating a plurality of toner images on the transfer band before transfer to the carrier material. A second transfer mode provides continuous transfer of the toner image to the carrier material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a printer or copier with a transferstation for simultaneous both-sided printing of a carrier material. Theinvention is also directed to a corotron device that can be utilized inthe transfer station.

2. Description of the Related Art

High-performance printers and high-performance copiers often have thecapability of printing the front side and the back side of a carriermaterial, for example paper. This operating mode is also called duplexprinting. It is known to first print one side, for example the frontside, with a toner image and to subsequently turn the carrier materialover. It is then reconveyed to the same printing station in order tothen print the second side, usually the back side, with a second tonerimage. This type of duplex printing is known both for web-shapedmaterial as well as for a single-sheet carrier material. In such aprinting mode, the overall throughput is not high due to the additionaltransport and the turn-over of the carrier material. Given another knownsolution, a printer or copier system is given two printing units,whereby each printing unit prints one side of the carrier material. Inthis case, considerable space for the two printer units is requiredwithin the system and the technological outlay is high.

Given a printer device disclosed by U.S. Pat. No. 5,526,107, continuousform paper is supplied to a transfer printing location of aphotoconductive cylinder that has electro-photographic units at twosurfaces for producing differently colored toner images. At the transferprinting location, the continuous form paper is printed with a firstcolor on the front side; subsequently, the continuous form paper isredirected and is supplied to a printing location at the samephotoconductive cylinder lying opposite the transfer printing locationand the back side is printed there.

European Patent Document EP-A-0 320 985 discloses that a transfer bandis employed, this carrying toner images that have been transferred froma photoconductive drum onto the transfer band. German Patent DocumentDE-A-197 13 964, which is identical in content with U.S. Pat. No.5,797,077, discloses a transfer station for simultaneous printing ofboth sides of a carrier material (duplex printing). The transfer stationcontains a pivotable transfer printing station that holds a transferband away from the carrier material in a first position, so that notoner images are transferred onto this carrier material. In thisposition, toner images are produced superimposed on the transfer band inorder to enable a multi-color printing. In a second position, thetransfer station is pivoted against the carrier material and transfersthe multi-color toner image.

Published PCT application WO 87/02792 discloses a corotron device havinga corotron electrode whose cooperating electrode is implemented as ametal plate. This metal plate lies at ground potential. The electricalfield generated between the corotron electrode and the cooperatingelectrode leads to a charge influencing of the toner particles.

SUMMARY OF THE INVENTION

An object of the present invention is to create a printer or copier thatenables a simultaneous printing of front side and back side of a carriermaterial given low outlay and with high printing quality.

This object is achieved by a printer or copier having a transfer stationfor the simultaneous both-sided printing of a carrier material, wherebya first endless transfer band of a first transfer module carries tonerparticles of a first polarity in the region of a transfer printinglocation, a second endless transfer band of a second transfer modulecarries toner particles of a second polarity in the region of thetransfer printing location, the carrier material is guided at thetransfer printing location between the first transfer band and thesecond transfer band, an electrostatic field is generated at thetransfer printing location that effects that the toner particles of eachand every transfer band separate from the respective transfer band as aresult of electrostatic forces and adhere to the surface of the carriermaterial lying opposite the respective transfer band, whereby eachtransfer module contains a switchable transfer printing station, that,in a first operating mode (a collecting and printing mode), initiallykeeps the respective transfer band at a distance from the carriermaterial, whereas a plurality of toner images are arranged on top of oneanother on the respective transfer band, and the carrier material doesnot move forward at the transfer printing location, and then conductsthe respective transfer band close to the carrier material in order totransfer the toner images arranged on top of one another there onto incommon, and that, in a second operating mode (a continuous printingmode) conducts the respective transfer band close to the carriermaterial in order to continuously print monochromatic toner images ontothe carrier printer.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained below on the basisof the drawings.

FIG. 1 is a schematic sectional view of an electrophotographic printerdevice for monochromatic and/or colored, single-sided or both-sidedprinting of a web-shaped carrier material, whereby the transfer stationof the invention can be utilized;

FIG. 2 is a schematic sectional view of a printer device according toFIG. 1 that prints the carrier material on both sides;

FIG. 3 shows schematically, an arrangement of critical parts of thetransfer station with charge reversal of the toner particles;

FIG. 4 is a detailed illustration of the arrangement according to FIG. 3for explaining the function;

FIG. 5 is an electrical equivalent circuit diagram that reflects theresistance conditions and conditions of the current at the transferprinting location;

FIG. 6 is a side view of an arrangement similar to that of FIG. 3 with anegative toner system;

FIGS. 7a, 7 b, 7 c and 7 d are electrical diagrams that showschematically, the possible relationships of potential at the transferprinting drums;

FIG. 8 is a side view of an exemplary embodiment wherein the transferbands partially wrap around the transfer drums;

FIG. 9 is a side view of an exemplary embodiment with guide rollers;

FIG. 10 is a detailed illustration of the arrangement according to FIG.9;

FIG. 11 is an arrangement similar to that of FIG. 9, whereby theelectrical field required for the transfer printing is built up betweenthe guide roller and the transfer drum;

FIGS. 12a and 12 b are an electrical equivalent circuit diagram directedto the exemplary embodiment according to FIG. 11 and a perspective viewof the drums;

FIG. 13 is an arrangement according to FIG. 11, whereby additionaldelivery rollers are provided;

FIG. 14 is an exemplary embodiment with deflection bows;

FIG. 15 is an enlarged side view showing the conditions of the currentin the exemplary embodiment according to FIG. 14;

FIG. 16 shows the exemplary embodiment according to FIG. 14 withinsulated deflection bows;

FIG. 17 shows an exemplary embodiment having electrically conductivedeflection bows that are conducted to ground potential via a resistor;

FIG. 18 is a side view of an exemplary embodiment similar to that ofFIG. 13;

FIGS. 19a, 19 b and 19 c are perspective views of a plurality ofexemplary embodiments for a transfer drum;

FIG. 20 id s perspective view of a transfer drum composed ofhigh-impedance material having lateral electrode terminals;

FIG. 21 is a perspective view of a transfer drum having an electricallyconductive core in a high-impedance coating;

FIG. 22a is a side view of a charge reversal corotron device having twocorotron wires and two cooperating electrodes fashioned as blades and

FIG. 22b is an enlarged detail view of the blades;

FIG. 23 is a detail view of a charge reversal corotron having a corotronwire and a blade utilized as a cooperating electrode, whereby the fieldlines of the effective electrical field are indicated;

FIG. 24 is a perspective view of a cooperating electrode that isexecuted as a blade;

FIG. 25 is an enlarged illustration of a portion of a blade, whereby theblade is serrated;

FIG. 26 is a perspective illustration of a cooperating electrode that iscomposed of an arrangement of individual pins;

FIG. 27 is a perspective illustration of a cooperating electrode that iscomposed of a wire; and

FIG. 28 is a side view of a transfer printing corotron device having acorotron wire and having a cooperating electrode fashioned as a blade.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following preferred embodiments are presented without limitation tothe scope of the claims.

According to the present invention, two transfer bands reside oppositeone another at the transfer printing location, the toner particlesthereof having different polarities. An electrostatic field is thengenerated that is directed such that the toner particles are repelledboth from the first transfer band as well as from the second transferband and agglomerate onto the respective surfaces of the carriermaterial. In this way, a simultaneous transfer printing is achieved. Thetransport path of the carrier material remains short since the carriermaterial need not be conducted past two printing stations or,respectively, past one printing station twice. Additionally, an interimfixing of the toner image which is transferred onto the carrier materialis eliminated, as a result whereof the technical outlay is reduced andthe printing quality remains high.

In a preferred exemplary embodiment, both transfer bands have tonerimages with toner particles of the same polarity in a section precedingthe transfer printing location as seen in the delivery direction of thecarrier material, whereby a transfer corotron is arranged preceding thetransfer printing location along one of the transfer bands, thisgenerating an electrical field that reverses the polarity of the tonerparticles on this transfer band as a result of the charge reversal. As aresult of these measures, a uniform toner system, for example a positiveor negative toner system with positive or, respectively, negativecharging of the toner image, can be employed for both transfer bands.Accordingly, the printing quality is nearly identical on both sides ofthe carrier material.

Another exemplary embodiment is characterized in that two transfer drumsreside opposite one another at the transfer printing location, and inthat a DC voltage is applied to the transfer drums that generates theelectrical field for the transfer printing of the toner particles. Thetransfer drums assure, on the one hand, a precise guidance of thecarrier material and of the transfer bands in the region of the transferprinting location. On the other hand, they enable the build-up of anelectrical field in the region of the transfer printing location in asimple way.

In practice, an exemplary embodiment has proven itself wherein two guideelements are arranged in front of the transferred drums as viewed in thedelivery direction of the carrier material, the transfer bands and thecarrier material being conducted between the two guide elements. In thisway, the transfer bands and the carrier material are guided along arelatively great path distance given mutual touching. The fogging effectis reduced at the transfer printing location since the toner particleshave only a slight or no spacing from the surface of the carriermaterial and a locationally exact transfer printing thus ensues.

According to another aspect of the invention, a corotron device isprovided, this corotron device can be advantageously employed inconjunction with the transfer modules.

For printing a final image carrier, for example paper, a transfer of atoner image existing on an intermediate carrier onto the final imagecarrier is undertaken mechanically, thermodynamically orelectrostatically. For an electrostatic transfer of the toner image froma photoconductive band onto an intermediate carrier or onto a finalimage carrier, the toner particles must have a certain voltagepotential. The electrostatic transfer of the toner particles ensues dueto forces in the electrical field and is based on a difference inpotential between the toner particles and the final image carrier ontowhich the toner image is to be transferred. The force as a result of theelectrical field must thereby be greater than the bonding forces withwhich the toner particles are held on the intermediate carrier for tonerimages from which they are to be transferred.

In electrographic printer and/or copier devices, dry toner particles areutilized for the electrographic transfer with a suitable voltagepotential, so that the transfer of the toner particles onto a materialcan be implemented without additional charge influencing of the tonerparticles in the printer or copier. When the final image carrier is tobe printed on both sides (duplex printing), the final image carrier mustbe turned over or a simultaneous or time-offset transfer of the tonerparticles ensues from both sides onto the final image carrier. In orderto realize the transfer without interim fixing of the toner imagetransferred onto the final image carrier, the toner particles on thefirst side of the intermediate carrier must have a difference inpotential compared to the toner particles of the second side.Preferably, the toner particles are reversed in charge from a positivevoltage potential to a negative voltage potential with reference to theground potential. The toner particles can thus be transferredsimultaneously or time-offset without interim fixing, being transferredfrom the intermediate carrier onto the final image carrier from bothsides. The toner particles on both sides of the final image carrierattract each other through the final image carrier due to theirdifferent potentials and/or are attracted by the difference in potentialcompared to the final image carrier, so that they adhere on the finalimage carrier.

After the transfer process, toner particles remain adhering to theintermediate carrier from which they are to be transferred, i.e. theyhave not been successfully transferred. This is thereby a matter oftoner particles of a few percent of the toner image, usuallysubstantially less than 20%. These toner particles that have not beentransferred usually have a low or an incorrect voltage potential. Inorder to carry out a further transfer of these untransfered tonerparticles, for example for cleaning the intermediate carrier, with ahigh efficiency, it is necessary to charge the toner particles to adefined potential. This charging event ensues with a corotron device.The intermediate carrier thereby forms the cooperating electrode for thecorotron device. When the intermediate carrier is of a conductivematerial having a specific resistance of less than 10⁶ ohms cm, then theintermediate carrier is applied to ground potential or to some othersuitable voltage potential and thus serves as a cooperating electrode.When the intermediate carrier, for example in the case of aphotoconductor, is provided with a light-sensitive cover layer whosedark resistance is an extremely high-impedance (for example, above 10⁶ohms cm), a cooperating electrode must be arranged at the back side ofthe intermediate carrier. Cooperating electrodes are preferablyimplemented as metal plates or as conductive deflection rollers. Sincedeflection rollers involve high mechanical outlay, increased spacerequirements and high costs, metal plates are mainly utilized as thecooperating electrodes.

The cooperating electrode should have a low transfer resistance comparedto the intermediate carrier. The intermediate carrier is conductedpassed the stationary cooperating electrode in a non-contacting fashion.In order to achieve the low transfer resistance, the intermediatecarrier must be conducted past the fixed cooperating electrode at aslight distance. This distance preferably amounts to 0.2 mm through 1.0mm. The forces between two bodies whose difference in potentialgenerates an electrical field are comparable to the forces between twoplates of a plate capacitor, whereby one plate of the plate capacitor isformed by the cooperating electrode and the other plate is formed by theunderside of the intermediate carrier. This force leads to deflection ofthe web-shaped intermediate carrier in the direction of the cooperatingelectrode at the cooperating electrode, so that the carrier touches andadheres to it. As a result of the contact between the moving web-shapedintermediate carrier and the stationary cooperating electrode, adhesionand sliding friction arise. The mechanical energy required in additionto the drive of the intermediate carrier due to this friction betweenthe intermediate carrier and cooperating electrode must be provided bythe drive unit of the intermediate carrier. Moreover, the intermediatecarrier and/or the cooperating electrode becomes worn as a consequenceof the sliding friction.

A corotron device is provided wherein low attractive forces occurbetween the intermediate carrier for toner images and the cooperatingelectrode and the charge carrier exchange is assured.

According to the new corotron device, the cooperating electrode hasconductive elevations whose end points project in the direction of thecorotron wire and that lie in a plane parallel to the longitudinal axisof the corotron wire. As a result of this fashioning of the cooperatingelectrode, what is achieved is that the attractive force between theintermediate and the cooperating electrode is substantially reduced.This attractive force is critically dependent on the effective area. Thecritical effective area is the area of the cooperating electrode facingtoward the intermediate carrier. As a result of the arrangement ofelectrically conductive elevations, whose end points represent thecritical effective area, it is assured that the effective area and,thus, the attractive forces between the intermediate carrier and thecooperating electrode are low. What is also achieved as a result of thisarrangement is that an intensified exchange of charge carriers as aconsequence of a spike discharge occurs due to the curvatures at theelevations.

A preferred embodiment provides that the elevations of the cooperatingelectrode are arranged along the longitudinal axis of the corotron wire.What is thereby achieved is that the electrical field for influencingthe charge of the toner particles is uniformly formed and thearrangement of the cooperating electrode is possible in a space-savingfashion. Another embodiment is characterized in that the cooperatingelectrode contains individual pins as elevations. What is therebyachieved is that the cooperating electrode can be cost-beneficiallymanufactured of standardized component parts.

Another preferred embodiment is comprised therein that the cooperatingelectrode contains acutely tapering elevations. What is thereby achievedis that the effective area of the cooperating electrode and, thus, theattractive force between the intermediate carrier for toner images andthe cooperating electrode is reduced further.

According to another aspect, it is provided in a corotron device for anelectrographic printer and/or copier device that the cooperatingelectrode is fashioned like a blade having a cutting edge, whereby thecutting edge is arranged in parallel to the longitudinal axis of thecorotron wire. What this development achieves is that, for example, asheet metal plate that is arranged perpendicular to the intermediatecarrier for toner images and proceeds over the width of the intermediatecarrier for toner images, is utilized as the cooperating electrode. Suchan arrangement is space-saving and cost-beneficial. What is alsoachieved by this arrangement is that an automatic exchange of chargecarriers (to a spike discharge) arises due to the curvatures at thecutting edge.

Another beneficial embodiment of the corotron device provides that thecutting edge of the cooperating electrode is serrated and that theserrations taper in the direction of the corotron wire, so that the endpoints and/or end surfaces of the serrations project in the direction ofthe corotron wire and lie parallel to the longitudinal axis of thecorotron wire. What is thereby achieved is that the effective area onwhich the amount of the attractive force between the intermediatecarrier for toner images and cooperating electrode is dependent isreduced compared to the continuous blade, as a result whereof theattractive force is reduced farther. The spike discharge is promotedfurther.

According to another aspect, it is provided in a corotron device for anelectrographic printer and/or copier device that the cooperatingelectrode is formed with a wire whose longitudinal axis is arrangedparallel to the longitudinal axis of the corotron wire. What is achievedwith this development is that, for example, a corotron wire is alsoprovided as a cooperating electrode. This wire proceeds over the widthof the intermediate carrier. Such an arrangement is space-saving andreduces the number of component parts utilized.

FIG. 1 shows a printer device for monochromatic and/or colored,single-sided or both-sided printing of a web-shaped carrier material,for example a paper web. The printer device is modularly constructed andhas a delivery module M1, a printer module M2, a fixing module M3 and apost-processing module M4. The delivery module M1 contains elements fordelivering a continuous form paper taken from a stacker to the printingmodule M2. This printing module M2 contains the transfer station thatprints the carrier material that is subsequently fixed in the fixingmodule M3 and cut and/or stacked in the post-processing module M4.

The printer module M2 contains the units required for printing aweb-shaped carrier material with toner images, these units beingarranged at both sides of a transport channel 11 for the carriermaterial 10. These units essentially comprise two differentlyconfigurable electrophotographic modules E1 and E2 with appertainingtransfer modules T1 and T2 that, together, form the transfer station T.The modules E1 and T1 are allocated to the front side of the carriermaterial 10; the modules E2 and T2 are allocated to the back side of thecarrier material 10.

The essentially identically constructed electrophotographic modules E1and E2 contain a preferably seamless photoconductive band 13 conductedover deflection rollers 12 and electro-motively driven in an arrowdirection that, for example, is an organic photoconductor, also referredto as OPC. The units for the electrophotographic process are arrangedalong the light-sensitive outside of the photoconductive band 13. Theseunits serve the purpose of generating toner images allocated to theindividual color separations on the photoconductive band 13. To thisend, the photoconductor 13 moving in the arrow direction is firstcharged to a voltage of approximately −600 V with the assistance of acharging device 14 and is discharged to approximately −50 V dependent onthe characters to be printed and with the assistance of a charactergenerator 15 composed of an LED comb.

The latent charge image generated in this way and situated on thephotoconductor 13 is then inked with toner with the assistance ofdeveloper stations 16/1 through 16/5. Subsequently, the toner image isloosened with the assistance of the intermediate illumination means 17and is transferred onto a transfer band 19 of the transfer band moduleT1 in an intermediate transfer printing region 18 with the assistance ofa transfer corotron device 20. Subsequently, the entire photoconductiveband 13 is discharged over its entire width with the assistance of thedischarge corona device 21 and is cleaned of adhering toner dust via acleaner device 22 having cleaning brushes. A subsequent intermediateillumination device 23 sees to a corresponding charge-wise conditioningof the photoconductive band 13 which, as was already set forth, is thenuniformly charged with the assistance of the charging device 14.

Toner images allocated to individual color separations are generatedwith the electrophotography module E1 or, respectively, E2, the totalityof these color separations forming the color image to be printed. Tothis end, the developer stations 16/1 through 16/5 are fashioned to beswitchable. They respectively contain the toner allocated to anindividual color separation. For example, the developer station 16/1contains black toner, the developer station 16/2 contains toner havingthe color yellow, the developer station 16/3 contains toner having thecolor magenta, the developer station 16/4 contains toner having thecolor cyan and, for example, the developer station 16/5 contains bluetoner or toner of a special color. Both single-component as well astwo-component toner/developer stations can be employed as developerstations. Preferably, single-component toner developer stations areutilized, these working with fluidizing toner as known, for example,from U.S. Pat. No. 477,106 (Applicant: Fotland). The subject matter ofthis U.S. Patent is likewise a component part of the present disclosureand is incorporated herein by reference.

In order to achieve the switchability of the developer stations, i.e. inorder to be able to individually actuate each individual developerstation, these stations, given employment of fluidizing toner, can befashioned in conformity with German Patent Application DE 196 52 866.The switching of the developer station accordingly ensues by changingthe electrical bias voltage of the transfer drum or, respectively, bychanging the electrical bias voltage of the applicator drum. It is alsopossible to switch the developer stations in that they are mechanicallyshifted and are thereby brought into contact with the photoconductiveband 13. Such a principle is known, for example, from German PatentDocument DE-A-196 18 324.

During operation of the printer device, a toner image that is allocatedto an individual color separation is respectively generated by a singledeveloper station with the assistance of the developer stations 16/1through 16/5. This toner image is then electrostatically transferredonto the transfer band 19 via the transfer printing device 18 incombination with the transfer corona device 20. The transfer module T1contains this transfer band 19, which is composed of a rubber-likesubstance, is conducted around a plurality of deflection devices and ismotor-driven. The transfer band 19 is fashioned as an endless band andwithout a seam similar to the photoconductor band 13. It is moved in thearrow direction, namely proceeding from the transfer region with thedrum 18 and the transfer corona device 20 to a transfer printing station24 with transfer drums, is moved therefrom further around the deflectionroller 25 to a cleaning station 26 and from the latter is in turn movedto the transfer region 18 and 20 with the deflection roller 27 arrangedthereat.

The transfer band 19 in the transfer module T1 serves as a collector forthe individual toner images allocated to the color separations that aretransferred onto the transfer band 19 via the transfer device 18 and 20.The individual toner images are thereby arranged above one another, sothat an overall toner image corresponding to the color image arises. Inorder to be able to generate the overall toner image and in order to beable to then transfer it onto the front side of the carrier material 10,the transfer module T1 contains a switchable transfer station 24. This,corresponding to the illustration in FIG. 1, can contain a plurality ofmechanical displaceable transfer printing drums 28 with appertainingtransfer printing corona device 29. In the operating condition of“collecting”, the transfer printing drums 28 and the transfer printingcorona device 29 are shifted upward according to the arrow direction, sothat the transfer band 19 is spaced from the carrier material 10. Theindividual toner images are taken from the electrophotographic module E1in this condition and are superimposed on the transfer band 19. Thecleaning station 26 is deactivated by being pivoted out. The carriermaterial 10 is at rest in the region of the transfer printing station 24in this operating condition.

The electrophotography module E2 and the transfer module T2 for thebackside of the recording medium 10 are constructed corresponding to themodules E1 and T1. Here, too, an overall toner image is generated on thetransfer band by collecting individual toner images for the backside,whereby the corresponding transfer printing station 24 is also pivotedaway here in the operating condition of “collecting”.

For a simultaneous printing of the front side and back side of thecarrier material 10, the transfer bands 19 of the transfer module T1 andT2 are simultaneously brought into contact with the carrier material 10in the region of the transfer printing stations 24 and the carriermaterial 10 is thereby moved. At the same time, the cleaning stations 26of the transfer modules T1 and T2 are pivoted in and activated. Aftertransfer of the two toner images onto the front side or, respectively,the back side of the carrier material 10, toner image residues adheringto the transfer bands 19 are removed by the cleaning stations 26. Thisis again followed by a collecting cycle for generating new toner images,whereby the transfer bands 19 are pivoted out and the carrier material10 is at a standstill. The transfer of the toner images from thetransfer modules T1 and T2 onto the carrier material 10 thus ensuesgiven a start-stop operation of the carrier material 10.

The carrier material 11 is moved in the paper transport channel with theassistance of motor-driven transport drums 38. In the region between thetransport drums 38 and the transfer printing stations 24, charging or,respectively, corona devices 39 can be arranged for paper conditioningso that the paper 10 is, for example, uniformly set in terms of chargebefore the transfer printing.

So that the carrier material 10 composed of paper does not tear giventhe start-stop mode and can also be continuously supplied, the deliverymodule M1 contains a loop forming means 30. This loop forming means 30functioning as a web storage buffers the carrier material 10 which iscontinuously taken off from a stack holding device 31.

After the transfer printing of the two chromatic toner images in theregion of the transfer printing stations 24 onto the carrier material10, these must still be fixed. The fixing model M3 serves this purpose.It contains an upper and a lower row of infrared radiators 32 betweenwhich the paper transport channel for the carrier material 10 proceeds.The toner image located both on the front side as well as on the backside of the carrier material 10 and fixed by the infrared radiators 32is still hot and soft and is guided free in non-contacting fashion overa deflection drum 33 arranged at the output side following the region ofthe infrared radiators 32. The fixing ensues with the heat generated bythe infrared radiators 32. A cooling of the carrier material 10 as wellas a smoothing, for example via corresponding decurler devices, ensuesin a cooling path with cooling elements 34 and deflection rollers 35following the infrared radiators 32. Lower-driven air chambers can serveas cooling elements 34. After the fixing of both toner images andcooling, a corresponding post-processing of the carrier material 10ensues within the post-processing module M4 that, for example, cancontain a cutter device 36 with stacking device 37.

A microprocessor-controlled control means ST coupled to the devicecontroller GS serves the purpose of being able to realize the variousoperating conditions, the control means ST being in communication withthe components delivery module M1, printer module M2 and fixing moduleM3 or, respectively, post-processing module M4 to be controlled andregulated. Within the modules, it is coupled to the individual units,thus, for example, to the electrophotography modules E1 and E2 and tothe transfer modules T1 and T2. A control panel B via which the variousoperating conditions can be input is connected to the device controllerGS or, respectively, to the control ST, which can be a component part ofthe device controller. The control panel B can contain a touch screenpicture screen or, respectively, a personal computer PC with a coupledkeyboard. The control itself can be conventionally constructed.

Given the embodiment according to FIG. 2, the electrophotography modulesE1 and E2 contain two devices B1 and B2 that work independently from oneanother and generate images. The first image generating device B1contains a character generator 15, a charging device 14, an intermediateillumination device 23, a cleaning device 22, a discharge corotrondevice 21 and a developer station 16/1. The second image-generatingdevice B2 is constructed analogous thereto with a charging device 14,character generator 15, a development station 16/2 and an intermediateillumination device 17. The developer station 16/1 can be allocated to afirst color, for example black, and the developer station 16/2 can beallocated to a second color, for example blue or some other color. Withthe assistance of the electrophotography modules E1 or E2, it is thuspossible to generate a first toner image having the color black and tosuperimpose a toner image having the additional color on this blacktoner image with the second image-generating device B2. The toner image(spot color toner image) superimposed is in this way is then transferredonto the transfer modules T1 and T2 and is transferred from the latterdirectly onto the carrier material 10. It is thus possible to applytwo-color toner images on both sides to the continuously moved carriermaterial 10. When only one of the image-generating devices B1 or B2 isactivated, monochrome printing is continuously carried out. In bothoperating modes, the transfer modules T1 and T2 serve merely for thetransfer of the toner images without needing the operating mode of“collecting”. However, one can also imagine that both image-generatingdevices B1 and B2 be actuated in alternation and the transfer modules T1and T2 in the operating mode of “collecting”, as initially set forth.

The transfer devices T1 and T2 shown in FIGS. 1 and 2 belong to thetransfer station T, whose critical parts are explained below withreference to FIGS. 3 through 21. FIG. 3 shows an exemplary guide exampleof the transfer station T, whereby two transfer drums are utilized. Thetoner image 44 for the front side of the carrier material 43 is locatedon the transfer band 41. The toner image 45 for the backside of thecarrier material 43, which is preferably a paper web, is located on thesecond transfer band 42. The two toner images 44 and 45 have, forexample, been transferred onto the transfer bands 41 and 42 with theassistance of the electrophotographic devices E1 and E2 according toFIG. 1. In the present instance according to FIG. 3, a positive tonersystem is employed, i.e. the toner particles have positive electricalcharges after the application of the toner images 44 and 45, asindicated in FIG. 3. The carrier material 43, which is conveyed in thedirection of the arrow P1, is located between the two transfer bands 41and 42. Two electrically conductive transfer drums 49 a and 49 b guidethe transfer bands 41 and 42 such that they touch the carrier material43. An electrical DC voltage U is applied to the transfer drums 49 a and49 b, the voltage being supplied from a DC voltage source 40. Thetransfer printing process ensues in the region of the transfer drums 49a and 49 b facing toward one another, whereby toner particles aretransferred from the transfer bands 41 and 42 onto the respectivesurface of the carrier material 43. This region is also referred to as atransfer printing location. A transfer printing corotron 47 a isarranged at the transfer band 42 preceding the transfer printinglocation, the corotron 47 a being supplied with negative DC voltagecompared to ground from a DC voltage source 48. A ground electrode 47 bresides opposite the transfer charge reversal corotron 47 a.

Fundamentally, the transfer bands 41 and 42 can be composed of aninsulating material or of a conductive material. The aim is that thetransfer bands 41 and 42 as well as the carrier material 43 have thesame surface velocities. Too great a relative motion of the surfacesrelative to one another would cause a mechanical smearing of the tonerimages 44 and 45 and could thus negatively influence the printingquality.

FIG. 4 shows the functioning of the simultaneous transfer printing givenemployment of a positive toner system. Due to the electrical fieldgenerated by the charge reversing corotron 47 a (shown in FIG. 3), thepolarity of the toner particles arranged on the lower transfer band 42is reversed, i.e. the toner particles 46 no longer have a positivecharge but a negative charge, as indicated in FIGS. 3 and 4. The tonerparticles of the toner image 44 continue to be positively charged. As aresult of the voltage U applied to the transfer drums 49 a and 49 b, anelectrostatic field F forms whose field lines proceed dependent on theshape of the transfer drums 49 a and 49 b, i.e. particularly dependenton the radius of curvature. It is indicated in FIG. 4 that theelectrical field F is largely uniform in the plane of the middle axes ofthe transfer drums 49 a and 49 b and becomes less uniform toward theedge along the plane of the carrier material 43. Dependent on theelectrical field strength that can be set with the voltage U, the tonerparticles of the upper toner image 44 separate from the transfer band 41and deposit on the front side of the carrier material 43. Since thepotential of the upper transfer drum 49 a is positive, a repellant forcederives for the toner particles of the toner image 44 that effects theagglomeration of the toner particles on the surface of the carriermaterial 43. The lower transfer drum 49 b has a negative voltagepotential referred to the potential of the toner particles 46 having anegative charge. Accordingly, these toner particles 46 are repelled fromthe surface of the lower transfer band 42, migrate opposite thedirection of the electrical field F to the back side of the carriermaterial 43 and agglomerate thereat.

Isolated toner particles can already separate early in the non-uniformregion, for example in the region of the field line F1, of theelectrical field F. Due to the inhomogeneity of the field and due to theincreased distance between the surfaces of the carrier material 43 andthe transfer bands 41 and 42, the point of incidence of the tonerparticles on the carrier material 43 is not exactly defined; a foggingeffect can occur that is known under the technical term of “fogging”.This effect is discussed in greater detail later.

FIG. 5 shows an electrical equivalent circuit diagram that is shown as acircuit having series resistors R. The flowing current i derives fromOhm's law, i.e. the current i is the quotient of the voltage U dividedby the sum of the individual resistors R. The aim is that the resistorsR of the two transfer drums 49 a and 49 b are as small as possible. Thiscan be realized with the assistance of conductive materials, i.e., forexample, transfer drums of metal are employed. It is also to be providedthat the resistors R of the transfer bands 41 and 42 are as large aspossible so that the overall current i remains small. Given a greatoverall current i, namely, the wear of the transfer bands 41 and 42 isincreased. The resistance R of the transfer bands 41 and 42 must,however, assume a finite value so that the electrical field F forms withhigh intensity at the surface of the respective transfer bands 41, 42.When, namely, the resistance R of the transfer bands 41 and 42 is toohigh, then the effective distance for the electrical field F isincreased; it extends from the surface of the transfer drum 49 a up tothe surface of the transfer drum 49 b. Given the same voltage U, thefield strength within the field F is then attenuated. Given a certainconductivity of the transfer bands 41 and 42, the effective distance forthe electrical field F between the transfer bands 41 and 42 is reducedand, thus, the field strength is increased given a voltage U thatremains the same.

FIG. 6 shows critical parts of the transfer station T given employmentof a negative toner system, i.e. wherein the charges of the tonerparticles are negatively charged after application onto the transferbands 41 and 42. The polarity reversal is again effected by the chargereversing core 47 a that, however, has positive potential in this case.Likewise, the transfer drums 49 a and 49 b are driven with a voltage U,so that an electrical field arises whose field strength reversescompared to the exemplary embodiment of FIG. 3. The function of thetransfer printing corresponds to that described up to now but merelywith an inverted operational sign of the charge and of the fieldstrength.

FIGS. 7a, 7 b, 7 c and 7 d show the possible relationships of potentialat the transfer drums 49 a and 49 b. One of the transfer drums 49 a and49 b is at ground in FIGS. 7a and 7 b. Likewise, one electrode of the DCvoltage source 40 is applied to ground. FIG. 7c shows a symmetricalvoltage drive whereby the voltage mid-point is applied to ground. FIG.7d shows an asymmetrical voltage drive for the transfer drums 49 a and49 b.

FIG. 8 shows a development of the arrangement according to FIG. 3. Thecarrier material 43 to be printed is guided such by the transfer drums49 a and 49 b that it wraps around the transfer drums 49 a and 49 b by arespective, predetermined wrap angle. In this way, the region whereinthe respective toner image 44 or, respectively, 45 lies against thesurfaces of the carrier material 43 is enlarged. Inhomogeneities of theelectrical field F at the edge thereof have a less pronounced effect;the fogging effect is reduced.

FIG. 9 shows an exemplary embodiment which is shown in detail in FIG. 10wherein two guide rollers 49 c and 49 d between which the transfer bands41 and 42 as well as the carrier material 43 are guided are arrangedpreceding the transfer drums 49 a and 49 b as shown in the feeddirection of the carrier material 43. The two guide rollers 49 c and 49d are applied to ground potential, whereas the voltage U for generatingthe electrical field is applied to the transfer drums 49 a and 49 b. Thetwo guide rollers 49 c and 49 d bring the transfer bands 41 and 42 intocontact with the carrier material 43 or, respectively, reduce thespacing to a minimum. When, given forward conveying, the toner images 44and 46 reach the non-uniform region (see FIG. 4) of the electrical fieldand the first toner particles are transferred onto the surface of thecarrier material 43, then the flight path for these toner particles isminimal or, respectively, equal to zero, and a topically exact tonertransfer ensues. The fogging effect is avoided in this way and a highprint quality is achieved.

FIG. 11 shows a modification of the exemplary embodiment according toFIG. 9. The lower transfer drum 49 b and the upper guide roller 49 c arecharged with a voltage potential such that the electrical field F takeseffect between the drum 49 b and roller 49 c. The transfer drum 49 a andthe guide roller 49 d are seated in an insulated fashion and have afloating potential. As a result of these measures, the electrical fieldF required for the transfer printing is effective over a longerdistance, so that the transfer printing process proceeds more gentlysince the effective area on which the transfer of the toner particlesfrom the transfer bands 41 and 42 onto the surface of the carriermaterial 43 ensues is enlarged.

FIG. 12 a schematically shows the physical relationships on the basis ofan electrical equivalent circuit diagram. When the specific materialresistance ρ of the transfer bands 41 and 42 employed is low, thenrelatively high current i result as a result of Ohm's law. Given apermanently applied voltage U, this can yield an undesirably highelectrical power P according to the relationship:

P=U·i.

Due to inhomogeneities of the materials for the transfer bands 41 and42, local current spikes are possible that allow the electrical field tobriefly collapse and, thus, disturb the process of transfer printing. Asa result of an increase of the spacing between the drums that form theelectrodes for the electrical field, the electrical resistance R of thetransfer bands 41 and 42 is increased, as is that of the carriermaterial 43. The current i that flows reduces correspondingly on thebasis of the relationship

R=ρ·l/A,

wherein R is the overall electrical resistance, ρ is the specificelectrical material resistance of the transfer bands, l is the effectivematerial length and A is the effective material cross-section, as shownin FIG. 12b.

FIG. 13 shows a combination of the exemplary embodiments according toFIGS. 9 and 11. Two delivery rollers 49 e and 49 f between which thetransfer bands 41 and 42 and the carrier material 43 are guided arearranged preceding the guide rollers 49 c and 49 d. The delivery rollers49 e and 49 f carry ground potential, whereas the arrangement of thedrums 49 a and 49 b and rollers 49 c and 49 d as well as the carrying ofthe potential thereof corresponds to that of FIG. 11. In this way, anelectrically neutral zone arises in the region of the delivery rollers49 e and 49 f, whereby the attractive forces of the toner particles withdifferent potential can be left out of consideration. A prematurejumping of toner particles in the region of increased distance betweenthe transfer bands 41 and 42 is thus avoided.

FIG. 14 shows a modification of the arrangement according to FIG. 9.Grounded deflection bows 49 g and 49 h are employed instead of thegrounded guide rollers 49 c and 49 d. These deflection bows 49 g and 49h can be arranged close to the transfer drum 49 a and 49 b, as a resultwhereof the length of the contact of the transmission bands 41 and 42with the carrier material 43 is shortened. When the arrangementaccording to FIG. 9 is compared to that according to FIG. 14, then itcan be seen that the minimum path wherein there is contact between thetransfer bands 41 and 42 and the carrier material 43 in FIG. 9 is thesum of the radii of the transfer drums 49 a or, respectively, 49 b andof the guide rollers 49 c or, respectively, 49 d. When velocitydifferences dv between the velocity of the transfer bands 41 and 42 andthe carrier material 43 occur, then this leads to a mechanical slip and,thus, to an undesired smearing of the toner images to be transferred.The smearing effect is all the greater the longer the contact path isor, respectively, the greater the velocity difference is. The reductionof the velocity difference between the transfer bands 41 and 42 and thecarrier material 43 is hardly possible in practice since lengthtolerances given pre-print forms must be compensated. In order tononetheless keep the smearing effect low, the length of the contactbetween transfer band 41 and 42 and the carrier material 43 is reducedaccording to the exemplary embodiment of FIG. 14 in that narrowdeflection bows 49 g and 49 h are employed whose sliding surfaces can bearranged close to the surface of the transfer drums 49 a and 49 b. Inorder to reduce frictional forces, it is meaningful to provide thedeflection bows 49 g and 49 h with a friction-reducing layer, forexample with a layer of a fluorine-containing plastic material, forexample PFA, ETFE, FEP, PFDC, Teflon or polyimide (PI). The surface wearof the deflection bows 49 g and 49 h can be reduced in that hard, wearresistant materials, for example chromium nickel steel, VA steel, areemployed or in that the deflection bows 49 g and 49 h are provided witha layer of a wear-reducing material, for example by nickel-plating, byemploying silicate or with the assistance of a surface hardening.

FIG. 15 shows the relations of the current given the example of FIG. 14,whereby the deflection bows 49 g and 49 h lie at ground potential. Theoverall current Iges derives from the sum of the currents Ium at thetransfer printing location and the quadrature-axis currents Iq1 and Iq2.The aim is that

Ium>>Iq 1+Iq 2

applies or that

Iq 1=Iq 2=0

applies. When the quadrature-axis current components Iq1 and Iq2, whichflow directly into the grounded deflection bows 49 g and 49 h throughthe transfer bands 41 and 42 are undesirably high, then the deflectionbows 49 h and 49 g can also be arranged so as to be electricallyinsulated, so that they assume a floating potential (see FIG. 16).

FIG. 17 shows an exemplary embodiment wherein the deflection bows 49 gand 49 h are electrically conductive but are connected to groundpotential via a resistor R. In this exemplary embodiment according toFIG. 17, too, the quadrature-axis current components are reduced.

FIG. 18 shows a modification of the exemplary embodiment according toFIG. 13. The delivery rollers 49 e and 49 f are replaced by deflectionbows 49 i and 49 j. These deflection bows 49 i and 49 j can beelectrically fashioned as indicated in the examples according to FIGS.16 and 17.

FIGS. 19a, 19 b and 19 c show various embodiments of the transfer drums.In FIG. 19a the transfer drum is cylindrically fashioned and fabricatedof an electrically conductive metal, being fabricated as a solidcomponent part. In FIG. 19b, the transfer drum is tubularly fabricatedof metal, i.e. is hollow on the inside. The FIG. 19c shows a metalliccore that can be composed of solid material or of a tube. This core isprovided with a cladding of high-impedance material. The employment of ametallic core for the transfer drum is expedient since it must befabricated very precisely with little out-of-roundness. In order tominimize concentricity errors, the circumference of the transfer drumand the length of the transfer band should have a whole-numbered ratiorelative to one another. The transfer bands, however, have a certainthickness fluctuation that has a disturbing influence on the transferprinting process; for example, a local detachment of the transfer bandsfrom the drum can occur. Advantageously, an elastic coating is thereforeapplied onto the transfer drum that can compensate slight mechanicaltolerances of the component parts on the basis of elastic deformation.This coating should have an electrical conductivity in order to be ableto build up a strong electrical field in the transfer printing zone atits outside skin. The electrical conductivity of the coating should liein the range from 0.5×10⁶ through 5×10¹² Ωcm but preferably in the rangefrom 0.5×10⁵ through 5×10⁹ Ωcm. The elastic coating should have a Shorehardness in the range from 10 through 90 Sh(A), preferably lying in therange from 20 through 70 Sh(A). 0.2 through 15 mm, preferably 0.5through 2 mm are to be set as thickness of the elastic coating. Theelastic coating can additionally have a layer of fluorine-containingplastic material, preferably of PFA, ETFE, FEP, PVDC or Teflon or can becomposed of a polyimide layer. The additional layer can also beelectrically insulating and have a maximum thickness of 40 μm,preferably 0.1 through 20 μm. The elastic layer can have conductivefillers, preferably lampblack, silicates, oxides added to it, thisenabling an increased layer thickness.

FIG. 20 shows a transfer drum that does not have a continuous metalliccore but lateral metallic contacting cylinders 50. The middle part 52 ofthe cylindrical drum is composed of a high-impedance material. Theresistance R over the length 1 of the drum is entered in the figure. Itcan be seen that the resistance R increases with increase length l, as aresult whereof the topical currents i drops over the length l when thevoltage U is applied. Different potentials thus derive over the lengthl, this being undesired.

FIG. 21 shows an exemplary embodiment of a transfer drum having alow-impedance, metallic core 56 on which a coating 54 that is composedof a relatively high-impedance material is applied. The resistance Rremains constant over the length l, as a result whereof a constantpotential also derives on the surface of the high-impedance jacketcoating along the length l. The core can also be manufactured of anelectrically conductive plastic, for example of the material PA thatcontains lampblack particles.

FIG. 22 shows a charge reversing corotron device 110 having two corotronwires 112 and having two cooperating electrodes 114 fashioned as blades.A photoconductor band 116 is provided as an intermediate carrier.However, a transfer band can also be utilized.

The photoconductive band 116 having a toner image 118 that has not yetbeen fixed and contains positively charged toner particles 120 or,respectively, negatively charged toner particles 122 after the chargereversal is conducted past between the two corotron wires 112 and thetwo cooperating electrodes 114, whereby it is guided and driven bydeflection rollers 124. The blades 114 are secured to a holder 126 thatalso produces the electrical connection to the ground potential of theprinter and/or copier device 128. The corotron wires 112 are surroundedby two shields 130 at that side facing away from the photoconductiveband 116. The photoconductor band 116 is conducted past the cooperatingelectrodes 114 at a distance in the range from 0.2 mm through 4 mm,preferably in the range from 0.2 mm through 1 mm. The negatively chargedtoner particles 122 of the latent toner image 118 are reversed in chargedue to the electrical field between the corotron wires 112 and thecooperating electrodes 114.

FIG. 23 shows a charge reversal corotron device 110 with a corotron wire112 and an individual blade utilized as a cooperating electrode 114,whereby the field lines 132 and 134 of the effective electrical fieldare indicated. The effective area, on which the amount of the attractiveforce between photoconductive band 116 and cooperating electrode 114 isdependent, is referenced 136. The cooperating electrode 114 has aconnection to a ground electrode. Alternatively, the cooperatingelectrode can have negative potential with reference to the groundpotential. An electrical field is formed between corotron wire 112 andthe cooperating electrode 114. This field 134 acts on the tonerparticles 122, which have a negative potential. The toner particles 122are discharged when they pass by the corotron wire 112 and are rechargedto a positive potential. The amount of the potential of what is nowpositively charged toner 120 is dependent on the dwell time of the tonerin the electrical field and on the density of the electrical field. Thephotoconductive band 116 is thereby attracted by the cooperatingelectrode 114. The attractive force F is calculated from therelationship:${F = \frac{{ɛ_{0} \cdot ɛ_{r}}{A \cdot U^{2}}}{2 \cdot d^{2}}},$

wherein ∈_(r) is the dielectric constant of the air betweenphotoconductive band 116 and the cooperating electrode 114, A is theeffective area 136 of the cooperating electrode 114 in the electricalfield, U is the difference in potential and d is the distance betweenthe underside of the photoconductive band 116 and the cooperatingelectrode 114.

FIG. 24 shows another cooperating electrode 114 that is implemented as ablade. This blade 115 has a rectangular cross-section and is secured inthe printer and/or copier 128 with a holder 126.

FIG. 25 shows a blade 114 whose cutting edge is serrated. The blade 114is arranged such in the printer/copier 128 that the serrations 140 taperacutely in the direction of the photoconductive band 116. The serrations147 are arranged at equal spacings. As a result of this arrangement, auniform charge reversal of the latent toner image 118 is assured. Theholder 126 of the blade 114 is not shown in this FIG. 25.

FIG. 26 shows a cooperating electrode 114 that is composed of anarrangement of individual pins 142. The pins 142 are arranged atsymmetrical spacings on a holder 126. The holder 126 is arranged such inthe printer and/or copier 128 that the ends of the individual pins 142lie in a plane that is parallel to the photoconductive band 116 andparallel to the corotron wire 112.

FIG. 27 shows a cooperating electrode 114 that is composed of a wire144. The wire 144 is arranged such in the printer and/or copier 128 witha suitable holding mechanism 126 that it lies in a plane that isparallel to the photoconductive band 116 as well as parallel to thecorotron wire 112. A shield 130 is arranged at that side of the wire 144facing away from the photoconductive band 116. A wire 144 similar to thecorotron wire 112 is utilized as the wire 144.

FIG. 28 shows a transfer printing corotron device 146 having a corotronwire 112 and a cooperating electrode fashioned as a blade 114. Twophotoconductive bands 116 a and 116 b are provided as the intermediatecarrier. Alternatively, however, two transfer bands could also beutilized. A toner image 118 a on the photoconductive band 116 a that hasnot yet been fixed contains positively charged toner particles 120. Atoner image 118 b on the photoconductive band 116 that has not yet beenfixed contains negatively charged toner particles 122. Thephotoconductive bands 116 a and 116 b as well as a paper web 148 areconducted past between the corotron wire 112 and the blade 114 withouttouching these, whereby the photoconductive bands 116 a and 116 b areguided and driven by deflection rollers 124. The drive and the guidanceof the paper web 148 is not shown in this figure. The corotron wire 122has a positive potential and the blade 114 has a negative potential withreference to the ground potential. The corotron wire 112 is surroundedby a shield 130 at that side facing away from the photoconductive band116 a. The positively charged toner particles 120 of the latent tonerimage 118 a are repelled by the positively charged corotron wire 112 andare attracted by the negatively charged toner particles 122 of thelatent toner image 118 b as well as by the negatively charged blade 114.Analogous thereto, the negatively charged toner particles 122 of thelatent toner image 118 b are repelled by the negatively charged blade114 and are attracted by the positively charged toner particles 120 ofthe latent toner image 118 a as well as by the positively chargedcorotron wire 112. The transfer printing corotron 146 exerts a force onthe positively and negatively charged toner particles 120 and 122 thatis greater than the bonding forces between the toner particles 120 and122 and the photoconductive bands 116 a and 116 b. The positively andnegatively charged toner particles 120 and 122 are transfer printed ontothe paper web 146 by the field forces of the electrical field. The tonerparticles 120 and 122 remain on the paper web 146 due to the bindingforces between the toner particles 120 and 122 and the paper web 146 aswell as due to the attractive force between the positively charged tonerparticles 120 on the one paper side and the negatively charged tonerparticles 122 on the other paper side.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

What is claimed is:
 1. A printer or copier, comprising: a transferstation for simultaneous both-sided printing of a carrier material; afirst transfer module at said transfer station, including: a firstendless transfer band operable to carry toner particles of a firstpolarity in a region of a transfer printing location; a second transfermodule at said transfer station, including: a second endless transferband operable to carry toner particles of a second polarity in theregion of the transfer printing location; apparatus operable to guidethe carrier material at the transfer printing location between saidfirst endless transfer band and said second endless transfer band; anelectrostatic field generator at the transfer printing location thateffects separation of the toner particles of each transfer band fromsaid respective transfer band as a result of electrostatic forces andcauses the toner particles to adhere to a surface of the carriermaterial lying opposite said respective transfer band; a switchabletransfer printing station in each of said first and second transfermodules, in a first operating mode, said switchable transfer printingstation initially keeps said respective transfer band at a distance fromthe carrier material so that a plurality of toner images are arranged ontop of one another on said respective transfer band and the carriermaterial does not move forward at the transfer printing location andthen conducts said respective transfer band close to the carriermaterial in order to transfer the toner images arranged on top of oneanother onto the carrier material in common, and in a second operatingmode, said switchable transfer printing station conducts said respectivetransfer band close to the carrier material in order to continuouslyprint monochromatic toner images onto the carrier printer.
 2. A printeror copier as claimed in claim 1, wherein said first operating mode is acollecting and printing mode, and said second operating mode is acontinuous printing mode.
 3. A printer or copier according to claim 1,wherein said first and second endless transfer bands, as viewed in adelivery direction of the carrier material, have toner images with tonerparticles of a same polarity in a section preceding said transferprinting location; and further comprising: a charge reversing Korotronarranged along one of said first and second endless transfer bandspreceding said transfer printing location, said charge reversingKorotron operable to generate an electrical field that reverses polarityof the toner particles on said one of said first and second endlesstransfer bands by charge reversal.
 4. A printer or copier according toclaim 3, wherein the toner particles in a section preceding said chargereversing Korotron are of a positive polarity.
 5. A printer or copieraccording to claim 3, wherein the toner particles in a section precedingsaid charge reversing Korotron are of a negative polarity.
 6. A printeror copier according to claim 1, further comprising: two transfer drumsdisposed opposite one another at the transfer printing location; and aDC voltage source applied to said two transfer drums to generate anelectrical field for transfer printing of the toner particles.
 7. Aprinter or copier according to claim 6, wherein one of said two transferdrums carries ground potential.
 8. A printer or copier according toclaim 6, wherein said two transfer drums have a symmetrical potential toground.
 9. A printer or copier as claimed in claim 6, wherein said twotransfer drums have an asymmetrical potential to ground.
 10. A printeror copier according to claim 6, wherein said two transfer drums arearranged such that the carrier material wraps said two transfer drums bya predetermined wrap angle.
 11. A printer or copier according to claim6, further comprising: two guide elements are arranged preceding saidtwo transfer drums as viewed in a delivery direction of the carriermaterial, said first and second endless transfer bands and the carriermaterial being guided between said two guide elements.
 12. A printer orcopier according to claim 11, further comprising: an electricalconnection to a first of said two transfer drums and a first of said twoguide elements lying diagonally opposite it to generate an electricalfield between said first transfer drum and said first guide element fortransfer printing of the toner particles.
 13. A printer or copieraccording to claim 12, wherein a second of said two transfer drums and asecond of said two guide elements are at a floating potential.
 14. Aprinter or copier according to claim 12, wherein said two guide elementsare rollers.
 15. A printer or copier according to claim 12, wherein saidtwo guide elements are rigid deflection bows whose sliding surfaces arearranged close to the transfer drums.
 16. A printer or copier accordingto claim 12, further comprising: two delivery elements disposedpreceding said two guide elements as viewed in the delivery direction ofthe carrier material, the transfer bands and the carrier material beingguided between said delivery elements.
 17. A printer or copier accordingto claim 16, wherein said two delivery elements have ground potential.18. A printer or copier according to claim 6, wherein said two transferdrums have a metallic core and an elastic coating having a predeterminedelectrical conductivity on said metallic core.
 19. A printer or copieraccording to claim 18, wherein said predetermined electricalconductivity of said elastic coating lies in a range from 0.5×10⁻⁶through 5×10¹² Ωcm.
 20. A printer or copier as claimed in claim 19,wherein said range is from 0.5×10⁵ through 5×10⁹ Ωcm.
 21. A printer orcopier according to claim 18, wherein said elastic coating has a Shorehardness in a range from 10 through 90 Sh(A).
 22. A printer or copier asclaimed in claim 21, wherein said elastic coating has a Shore hardnessrange from 20 through 70 Sh(A).
 23. A printer or copier according toclaim 18, wherein said elastic coating has a thickness from 0.2 through15 mm.
 24. A printer or copier as claimed in claim 23, wherein saidelastic coating has a thickness from 0.5 through 2 mm.
 25. A printer orcopier according to claim 18, wherein said elastic coating includes anadditional coating of fluorine-containing plastic material.
 26. Aprinter or copier as claimed in claim 25, wherein said additionalcoating is of a material selected from the list consisting of: PFA,ETFE, FEP, PVDC, Teflon and polyimide.
 27. A printer or copier accordingclaim 25, wherein said additional layer is electrically insulating andhas a maximum thickness of 40 μM.
 28. A printer or copier as claimed inclaim 27, wherein said additional layer is of a thickness of from 0.1through 20 μm.
 29. A printer or copier according to claim 25, whereinsaid additional coating has a conductive filler added to it.
 30. Aprinter or copier as claimed in claim 29, wherein said conductive fillerof said additional coating is one of lampblack, silicates, and oxides.31. A printer or copier according to claim 18, wherein said elasticcoating has conductive filler added to it.
 32. A printer or copier asclaimed in claim 31, wherein said conductive filler is selected fromlampblack, silicates, and oxides.
 33. A printer or copier according toclaim 1, wherein the carrier material is a band material.
 34. A printeror copier as claimed in claim 33, wherein said band materials is one ofa paper web and single sheets.
 35. A printer or copier according toclaim 1, wherein each transfer module contains a corotron device.